How to Control White Mold on Beans

Easy dissemination and difficult control characterize white mold on beans, an aggressive disease caused by a fungus Sclerotinia sclerotiorum, capable of resulting in losses of up to 100%. Integrating genetic, cultural, biological methods and the preventive application of fungicides, preferably in a preventive manner, are the main strategies to minimize losses.   

White mold on beans, a disease caused by the fungus Sclerotinia sclerotiorum (Lib.) de Bary, is widely disseminated in the regions that produce the most diverse crops in the country. It is a polyphagous fungus and can cause disease in many plant species.

White mold is also known by the common names stem rot, white rot, watery rot, sclerotinia rot, and sclerotinia wilt. This disease was considered to be common in regions with a cold to temperate climate, but in Brazil it is widespread in many areas and has occurred in a widespread manner in most regions producing beans and other host crops. With the increase in incidence and severity, it tends to cause, under ideal conditions for its development, losses of up to 100% if control measures are not adopted.

The contamination of Brazilian soils by S. sclerotiorum reached this situation due to the increase in the inoculum potential of this fungus, in the form of a resistance structure, the sclerotia. Furthermore, this pathogen is able to adapt to adverse environmental conditions. Its occurrence and severity have been greater in winter crops, under irrigation, or in crops when there is a mild climate and rainy periods, with an increase in relative and soil humidity.

The use of seeds carrying the pathogen contributed to the introduction and spread of the disease, aided by the movement of machines and implements from one contaminated soil to another, without proper cleaning and disinfection, among other management methods not adopted over time by many farmers.

Bean cultivars have not shown resistance to the pathogen S. sclerotiorum, which, once introduced into the area, is difficult to control. Its resistance structure, the sclerotium, can survive for many years under adverse conditions, in addition to being privileged with a wide range of hosts, including some weeds and being transmitted by seeds.

Development of the disease

the pathogen S. sclerotiorum It is a polyphagous fungus, it can cause disease in more than 200 genera of plants, covering 408 species, from crops with high economic expression to low expression, in addition to some weeds.

the fungus S. sclerotiorum it can develop throughout the aerial part of the plant and produce numerous sclerotia, externally and internally in the tissues. The formation of sclerotia is favored in an acidic environment. During the bean harvesting process, many of these sclerotia fall to the soil, contaminating it and remaining viable for many years, even in adverse conditions.

Normally, when the bean crop reaches the closing phase and the climatic conditions are low temperature, high relative and soil humidity and in the presence of sclerotia of S. sclerotiorum, the formation of reproductive structures, apothecia and ascospores, may occur.

Ascospores are ejected at maturity and may reach susceptible plant tissues and initiate the disease process. When the fungus reaches the flower petals, it will find nutrients that favor its development, and can progress to colonize other parts of the plant and even cause its death. Ascospores can also cause disease in other tissues, in addition to the petals.

Another way in which the disease begins is the formation of white mycelium with a cottony appearance, from sclerotia present in the soil or in cultural remains or introduced through concomitant contamination with the seeds. Upon reaching the plant tissue, infection and colonization begin.

the fungus S. sclerotiorum it can also be introduced into an area, over short and long distances, by the transmission of sclerotia in concomitant contamination of the seeds and in the form of dormant mycelium in the seeds. Rain and irrigation water can also spread the pathogen.

Ascospores of S. sclerotiorum coming from neighboring plants or crops also, upon reaching the plant tissue, initiate infection and colonization. Ascospores can be spread by wind over long distances.

SYMPTOMS

the fungus S. sclerotiorum can cause symptoms throughout the aerial part of the bean plant. The ideal conditions are high relative and soil humidity and temperatures between 20ºC and 25ºC, which can range from 5ºC to 30ºC.

Initially, at the point of infection, small lesions with a soggy appearance appear, progressing to the development of white mycelium, resembling cotton, on the bean tissue. These lesions progress, reaching extensive areas, where the formation of white mycelium continues, and then resistance structures, sclerotia, form internally and externally on the stems and pods. The acidic environment becomes conducive to the development of sclerotia. When the tissue affected by S. sclerotiorum becomes older or drier and has a light straw color, which stands out in the field.   

INTEGRATED MANAGEMENT

Controlling the disease is complex, as in addition to there being no bean cultivars with good levels of genetic resistance available, the fungus S. sclerotiorum presents two ways of attacking plants. By air, by the ascospores produced in apothecia, which are formed in the sclerotia, and through the soil by the sclerotia, which germinate and begin the process of their life cycle. The parts affected by the fungus, which fall into the bean tissues or the soil and the mycelium reaching the plant, can also initiate infection and colonization.

To control white mold, integrated management must be carried out, as only a few measures do not lead to the desired combat. In most situations, fungicides are included.

In integrated management, all possible control measures must be adopted.

Genetic method

It is the ideal control, efficient, economical and easy to adopt, however, as there are no bean cultivars with ideal genetic resistance to Sclerotinia sclerotiorum, it is necessary to give preference to cultivars with an upright shape. The erect cultivar, sown with greater spacing, facilitates aeration in the crop, providing an unfavorable environment for the development of the pathogen.

Cultural method

Understands several techniques. It is important that soil contaminated with S. sclerotiorum be prepared in order to eliminate the pathogen, through deep plowing to bury the contaminated cultural remains and sclerotia and thus their degradation occurs.

It is essential to always use certified seeds free from S. sclerotiorum, avoiding its introduction and/or dissemination in the area. There are also products for seed treatment.

Crop rotation, mainly with grasses and green manures, is ideal for reducing the inoculum potential in the soil, increasing the population of microfauna and microflora, beneficial parasites, antagonists and competitors with the pathogen, in addition to improving the physical and chemical structures and biological and provide greater aeration in the soil.

Direct planting in straw, such as brachiaria, has provided good results in reducing the incidence of white mold due to the physical barrier that prevents the entry of light, necessary for the formation of apothecia and the dissemination of ascospores.

Choosing the planting time to avoid the occurrence of white mold in the most vulnerable stage of the crop is an important technique to avoid losses caused by this disease.

Reducing planting density can reduce humidity, through greater ventilation inside the crop, which is unfavorable for the development of the disease.

When white mold occurs, reduce irrigation and improve drainage, keeping the humidity of the soil and the crop environment low.

Plant nutrition is important, as a well-nourished plant maintains greater vigor and natural resistance, but excess nitrogen must be avoided, which can leave the bean plant more vulnerable to attack by S. sclerotiorum.

Adopt the procedure to clean, wash and disinfect tools, boots and utensils, tractors and implements used in areas contaminated with S. sclerotiorum with disinfectant chemicals at each operation.

Biological method

The sclerotia of S. sclerotiorum can be parasitized by fungi such as Trichoderma spp., Coniothyrium minitans, Sporidesmium sclerotivorum, Aspergillus spp. etc., being more used Trichoderma spp., currently. Some isolates from Trichoderma sp. they can also promote better plant growth. Bacillus subtilis It has also shown effectiveness in controlling the disease.

chemical method

The application of fungicide should preferably occur preventively. It is important to reach the target, sclerotia and apothecia in the soil, the entire aerial part of the plants, with adequate coverage and carry out repetitions as necessary indicated by monitoring, following the manufacturer's recommendations.

The period after flowering, when the culture closes, is the most favorable phase for the beginning of the development of white mold. Therefore, at this stage, you must pay extra attention to the crop and climatic conditions, so as not to miss the moment to start applying fungicide.

Margarida Fumiko Ito, Agronomic Institute - IAC

Article published in issue 204 of Cultivar Grandes Culturas.